If you've got Google Now setup, you can just long-press the Home button and it will come up. Then you just say "OK Google, send a text message to ..." and continue. There is also a trick to be able to say "OK Google" from your home screen or anywhere. Quick tip: Get 'OK, Google' everywhere right now - CNET
Keep in mind that from inside any text message app or anywhere you can bring up the keyboard, there is a Microphone icon in the lower right corner.